Rewriting Bun in Rust
- ID
- 3494
- Status
- summarized
- Published
- 09 Jul 2026, 7:57 AM
- Fetched
- 09 Jul 2026, 8:26 AM
- Provider
- Simon Willison
- Category
- developer-ai
- Original URL
- https://simonwillison.net/2026/Jul/8/rewriting-bun-in-rust/
- Source URL
- https://simonwillison.net/atom/everything/
Summary
- Score
- 9.0
- Created
- 09 Jul 2026, 8:26 AM
- Tags
- Audience
- developersvibe_codersai_agent_usersai_ml_learners
What happened
Jarred Sumner details rewriting the Bun JavaScript runtime from Zig to Rust, a massive undertaking largely enabled by AI coding agents. The existing TypeScript test suite acted as a conformance harness, allowing an agent-driven port to pass a high percentage of tests within days, leading to a merge after about 11 days of monitoring and adversarial review. The Rust port has been live in Claude Code for nearly a month with minimal disruption.
Why it matters
This is a concrete, high-profile example of AI agents tackling a large-scale rewrite that was previously considered impractical. For builders in Malaysia and elsewhere, the key takeaway is that a strong, language-independent test suite plus adversarial review can make agent-authored code merges credible, even at the million-line scale. It also signals that language choice may no longer be a one-way decision for ambitious projects.
Discussion angle
How the Bun test suite became the real enabler of this agent-driven rewrite, and what Malaysian teams can learn about investing in conformance tests before attempting large AI-assisted migrations.